Validate that there is a problem you are solving for both sides of the marketplace. Learn which one of the two sides is more 'hungry' for your solution. Then focus on the one side that is less hungry. It sounds counter intuitive but it's really quite simple. It'll always be easy to get the side that is 'starving' to use your product /service but not true for the opposite.
Example: UBER. It's pretty obvious that any taxi driver who'll be promised higher fulfillment rates (customers) consistently over time will want to use UBER. Their survival (pay check) depends on it! What's not obvious is that folks would pay a premium to get a cab under 5 minutes. Uber had to build the initial fleet of cars in one city but then the focus turned to validating that there was demand for the can-ordering service before scaling supply further.